In the field of foundations it is often required to have excavations having a large diameter, at great depth and with minimal deviations with respect to their vertical axis. An example of application in which such excavations are required consists of making impermeable partitions carried out through intersecting piles. In these cases the guarantee of actual interpenetration of the primary and secondary piles, closely linked to the verticality of the excavations, is an essential condition to carry out the work correctly. The uncertainty of the verticality of the pile leads to onerous corrective choices, the most obvious of which is to reduce the pitch between the axes of the intersecting piles so as to compensate, with greater interpenetration, the possible deviations that can be created between adjacent piles. Of course, this translates into over-consumption of cement mixture and into longer work times in making a partition of known length.
The use of a guide tube to drive to the bottom of the excavation, which can act as a guide for the excavation tool, ensures better verticality of the pile. This is due to the much more rigid configuration of the tube with respect to that of a battery of telescopic rods or of a continuous helix, and the greatest advantages are obtained in the case in which layers of earth of very variable conformity and hardness are crossed. The use of the guide tube, (generally called “casing”), due to the high friction that is generated with the walls of the excavation, requires greater torques and greater pull-push forces at the excavation machines. In particular, such friction increases as the length and diameter of the guide tube increase. This means that above certain diameter and depth values it becomes disadvantageous to make a single machine that performs both the driving of the tube, and the excavation, since such a machine would have to be too big and cost too much. The use of external apparatuses connected to the excavation machine can allow greater diameters and tubing depths, but it greatly limits the mobility and speed of the excavation machine, as well as increasing costs.
Known machinery for making tubed piles can be substantially split into two categories, as a function of the depth of the pile. In order to make piles of medium-low depth, quantifiable in the value of 30-35 meters at most, it is foreseen to use a tracked machine equipped with a vertical tower, along which two rotary tables, commonly called “rotaries”, can slide, one on top of the other, in a constrained or independent manner. The two rotary tables both translate on the same sliding guides present in the tower. The upper rotary table sets a helix in translation and in rotation, said helix being equipped in its lower part with a tip with excavation teeth and has a length substantially equal to that of the tower. The lower rotary table sets a coating tube in translation and in rotation, usually in the opposite sense of rotation to that of the helix. The tube and the lower rotary table have a diameter such as to make the helix transit inside them, actuated by the upper rotary table. The tube is equipped with blades in its lower part and in its thickness in contact with the ground, so as to separate, while moving forward, a core of ground that will later be broken up and lifted by the helix above. The broken up ground is loaded by the auger of the helix and sent outside of the excavation.
The tube has a maximum installable length that is substantially less than that of the helix and that can be determined by subtracting the length of the rotary table that moves the tube itself from the length of the helix. The lower rotary table, commonly called “tubing device”, can generally have a length of about 3 meters. As a result, when the pile is finished, the tubed part represents a fraction of the total length of the excavation, generally not more than ⅔. It is not foreseen, in this type of equipment, to join additional tube or helix elements as the excavation progresses. Consequently, the depth reachable by the helix corresponds to about the length of the tower of the machine and the depth reachable by the tube depends on the maximum loadable length below the lower rotary table.
It is difficult for the maximum depth to exceed 30 meters, because for greater depths the machine would have to have a tower that is too long, which would be too heavy for the machine and could cause instability. On the other hand, it would be necessary to make extremely heavy and bulky machines, but becoming incompatible with all urban works where the spaces available are small. Moreover, a machine with such a long guiding tower would be difficult to transport. As the length of the tube increases, the thrust required to drive it also increases, but such a thrust must be limited based on the weight of the machine, which otherwise would tend to lift at the front. A greater tubed depth implies a greater weight of the battery of tubes and thus requires a greater extraction force of the machine, but also such an extraction force must be limited based on the size of the machine and the resistance of the tracked undercarriage. The maximum usable diameter for the tube depends on the maximum torque able to be delivered by the lower rotary table and also this must be limited based on the torsional resistance of the tower. Such resistance depends on the section and on the thicknesses of the tower. Also in this case, by exceeding certain limit values, the tower would be too heavy.
The driving of a tube having a diameter equal to 1200 millimeters to a depth of 20 meters seems to represent, as things stand, the performance limit that can be obtained by a single machine with two rotary tables. The advantageous aspects of this type of machinery (“cased secant piles” or CSP) for shallow excavations consist of the fact that the machine is relatively light and thus easy to manoeuvre and transport, it does not have support structures at the excavation, such as casing oscillators, and it moves autonomously within the worksite from one point of construction of the pile to another without the help of external transportation means. Moreover, the excavation can take place dry, without the addition of stabilizing liquids to support the walls. The absence of recycling means of such liquids, associated with the absence of vibrations, makes these CSP machines particularly suitable for use in urban settings. The addition of the cement mixture takes place through a conduit inside the shaft of the helix, with the help of an external pump. The extraction of the tube is preferably concurrent to the filling of the hole, so that the pressure exerted by the mixture can prevent the collapse of the walls no longer supported by the tube. In some cases it is possible to extract the tube at the end of filling the hole.
In order to make piles of greater depth, greater than 30/35 meters, a tracked machine with a vertical tower is generally used, along which a single rotary table moves on suitable guides. The rotary table sets a battery of telescopic rods in rotary movement, at the base of which there is an excavation tool, like for example a “bucket” or a drill. This technology, called LDP (acronym for “large diameter pile”) is generally used to make deep non-secant piles, where the limitations required for the deviation from verticality are less stringent. The use of telescopic rods makes it possible to reach much greater excavation depths with the tool with respect to the length of the tower on which the rotary table slides. LDP technology foresees that the final depth is obtained through repeated partial excavations, each of which involves the driving of the tool in the ground and results in an advancement equal to the length of the tool itself. Each partial excavation is obtained by applying a thrust and a rotation on the tool and, when the tool is full, the operator lifts it up from the bottom of the excavation until it is brought above the terrain surface, where it is emptied beside the machine, onto the ground or into a truck.
A drawback of LDP technology consists of the fact that, as the depth reached increases, the duration of the active excavation step, i.e. that for filling the tool, is increasingly short in proportion to the inactive steps of descent and ascent in the excavation. Another drawback is the fact that the pile is usually excavated with the addition of stabilizing materials that prevent the hole from collapsing, such as bentonite or polymers. The use of such stabilizers requires rather complex logistics and apparatus to obtain their recovery and recycling, like for example decanting and containment tanks, sieves, grit separators, etc. These apparatuses are difficult to adapt to use in tight urban spaces or in worksites that extend for many kilometers, requiring continuous movement of the equipment.
The alternative to using stabilizing substances is to use, in combination with LDP technology, a coating guide tube that can support the walls of the hole, preventing it from collapsing. The use of the tube is particularly advantageous when excavating below the water table, since it manages to keep the outflow of ground water inside the excavation to acceptable levels. In this case, excavation is carried out “dry” and there is less need for logistics linked to stabilizing fluids. If the section of hole to be tubed has a limited depth, and in any case compatible with the power of the machine, it is possible to use the rotary table itself, mounting a hauling extension (cup) beneath it, which couples with the tube, to rotate and thrust the tube in the ground. Due to the axial bulk of the telescopic rods, which cannot extend above the head of the guiding tower, the free space for the positioning of the tube beneath the hauling extension is limited to a few meters, in general not more than six or seven. As a result, being forced to use short tubes, even for limited tubed depths it is necessary to drive in one piece of tube at a time, joining it to those already driven in. Therefore a lot of time is spent fixing together the pieces of casing tube, with spanners and bolts that are usually locked by hand.
When the depth and/or the diameter to be made become high, the torque delivered by the rotary table of the machine is insufficient and external apparatuses become necessary, distinct from the machine, to drive the tube segments by rotation and thrusting up to the desired depth and to extract them at the end of the excavation. These apparatuses are usually bulky, heavy and expensive. The external apparatuses most commonly used are casing oscillators or “rotators” (full-rotators). These apparatuses are mainly made up of a monolithic base frame and a second upper frame that is moveable with respect to the first. Both of the frames develop about a central circular passage of large diameter, completely surrounding it. Such a central passage makes it possible to introduce a tube segment from above, crossing the frames, in order to drive it into the ground. Such apparatuses must therefore be positioned at the front of a common pile driving machine, at a lower height with respect to the base of the tower of the machine and aligning their central passage on the drilling axis of such a machine. Such apparatuses are equipped with suitable actuation means that connect the moveable upper frame to the base frame, allowing the upper frame to be made to perform vertical translations and rotations about the vertical axis of the central passage. Once the upper frame, through temporary gripping means, is able to transfer these movements to the tube to be driven. During its limited axial movement, the upper frame is not guided by any structural element of the apparatus, but only by the actuators and by the tube itself. In the casing oscillators the base frame rests directly on the ground. The upper frame is equipped with hydraulic clamps or jaws to grip or release the tube. All of the actuators of the clamp are usually fed by the hydraulic system of the pile driving machine. The thrusting takes place through hydraulic cylinders that bring the upper frame towards the base frame, whereas the rotation takes place, with partial and alternate movements, through a pair of hydraulic rotation cylinders mounted opposite one another. For every partial rotation it is necessary for the jaws to grip the tube, for the rotation cylinders to carry out their limited stroke, for the jaws to release the tube and for the rotation cylinders to carry out a reverse stroke to go back into the start of rotation condition. Therefore, very long cycle times are needed to carry out the excavation.
A “rotator” in brief consists of a rotary table with a passage having a large diameter, which constitutes an upper frame and which is moveable with respect to a monolithic base frame that also extends around the passage of the table to allow the insertion of the tube. The base frame rests on the ground. The rotary table comprises a through sleeve on which geared motors are fitted that allow the rotation thereof. Such a sleeve is provided with hydraulic jaws that wrap around the tube to be driven on its outer surface, transmitting the rotation to it only by means of the friction between jaws and tube. Through hydraulic cylinders that connect the upper rotary table to the base frame it is possible to generate small and limited vertical movements, always less than one meter, and thus exert a thrust or a pull on the tube. The limited vertical movement of the upper frame is not, however, guided by a tower or by elements of the frame, but exploits just the rigidity of the actuators and of the tube itself. In particular, the axial movement is limited because the axial stroke available is always less than the length of the piece of tube that is joined. In some variants, the “rotator” can comprise an autonomous power unit to supply its own actuators. In rare cases the “rotator” is connected to the hydraulic system of the pile driving machine.
The aforementioned external apparatuses for driving such tubes have numerous limitations and drawbacks. Firstly, the cylinders of both types of external apparatuses have limited strokes in the vertical direction, generally of the order of 400-600 millimeters, with consequent limited driving or extraction movements. In particular, the moveable part of these apparatuses, i.e. that capable of transmitting the thrust and the torque, even in the condition of maximum vertical stroke always remains at a height lower than the base of the tower of the machine. This is generally due to the substantial bulk of such apparatuses in the radial direction with respect to the excavation axis. Often, in order to allow the connection of such apparatuses to the machine it is necessary to dismount the lower segment of the tower of the machine. Strokes of greater width could lead to interference or collisions between the mobile part of the external driving apparatuses and the tower of the machine. As a result, in order to drive or extract a few tens of meters of tube a very large number of manoeuvres are needed, each of which comprises the steps of gripping, of translation and of release of the tube, and therefore takes a long time. A second limitation is due to the fact that the aforementioned external apparatuses, gripping the tube laterally through the upper frame, are not able to completely drive the tube until it is flush with the ground surface. In particular, the tube will always extend vertically above the base frame by a minimum amount sufficient to allow it to be gripped laterally. The tube, therefore, always extends at least partially inside such frames of the external apparatuses and, due to the fact that these frames are monolithic and completely surround the tube, the external apparatuses are fixedly connected to the driven tube, not being able to translate horizontally with respect to it. The aforementioned apparatuses, which actively operate only during the driving or extraction steps, are forced to remain on the axis of the pile even during the steps of casting and insertion of the cage that does not involve them. During the inactive steps, the driving apparatuses cannot be moved and exploited on other piles, unless they are lifted through a crane to axially disengage from the driven tube. This solution is, however, complex and not cost-effective.
A further limitation of casing oscillators and of “rotators” is due to the fact that their hydraulic jaws transmit the torque by clamping the tube on its outer surface, only by friction, and this requires the use of very thick tubes or ones with a double wall to prevent it from becoming oval. These tubes are particularly heavy and expensive.